A metazoan is, simply, an animal. The term comes from the Greek “meta” (after or beyond) and “zoa” (living things), and in modern biology it encompasses every member of the kingdom Animalia, from sponges clinging to rocks on the seafloor to blue whales cruising the open ocean. What unites this staggeringly diverse group is a set of shared biological features: all metazoans are multicellular, all are eukaryotes (their cells contain a nucleus), all are heterotrophs (they get energy by consuming other organisms rather than making their own food), and all develop from an embryo. The word “metazoan” shows up most often in evolutionary biology and paleontology, where scientists need a precise term that covers the entire animal kingdom without the everyday baggage of the word “animal.”
What Actually Sets Metazoans Apart
Plenty of organisms are multicellular. Plants, fungi, and many algae all build bodies from multiple cells. What distinguishes metazoan multicellularity is how those cells are organized. Animal cells lack rigid cell walls, and instead they stick together and communicate through a specialized toolkit of adhesion molecules and signaling proteins. One of the most important of these systems is integrin-mediated adhesion, which allows cells to attach to their surroundings and to one another while relaying mechanical and chemical signals. Research into the evolutionary origins of this system has traced its molecular components back to the single-celled ancestors of animals, suggesting that the building blocks for animal-style cell communication existed before multicellularity itself evolved.1PubMed Central. Ancient origin of the integrin-mediated adhesion and signaling machinery
Another hallmark is the extracellular matrix, a meshwork of proteins that surrounds and supports animal cells. Collagen is the signature protein here, and one particular form, collagen IV, is found in the basement membranes of essentially all animals investigated to date. Basement membranes are thin, sheet-like structures that underlie epithelial tissues (the layers of cells that line surfaces and cavities throughout your body). Research has concluded that collagen IV was a primordial component of the extracellular environment and that it enabled the very first assembly of multicellular animal tissues.2eLife. Collagen IV and basement membrane at the evolutionary dawn of metazoan tissues A related study found that the gene pair encoding collagen IV first appeared in the earliest-branching animal lineages, comb jellies and cnidarians (jellyfish, corals, anemones), and has been conserved across metazoans ever since.3PubMed Central. Collagen IV of basement membranes: I. Origin and diversification of COL4 genes enabling metazoan multicellularity, evolution, and adaptation
Beyond structural glue, metazoans share core signaling pathways that coordinate how cells grow, divide, differentiate, and die. The Notch and Wnt pathways, for example, are present in all multicellular animals, including sponges, but are absent in fungi and single-celled organisms.4PubMed Central. The interaction of Notch and Wnt signaling pathways in vertebrate regeneration Programmed cell death, or apoptosis, is another shared feature. Different animal lineages have evolved their own variations on the molecular machinery that carries out cell suicide, but the core capacity to selectively destroy cells is a deeply conserved metazoan trait.5Genome Biology and Evolution. Evolution of Apoptotic Signaling Pathways Within Lophotrochozoans
The Closest Living Relatives of Animals
If metazoans share all these traits, where did they come from? The best clues come from choanoflagellates, a group of single-celled (and sometimes colonial) organisms that are the closest living relatives of animals. Choanoflagellates are tiny aquatic creatures, each bearing a single whip-like flagellum surrounded by a collar of microvilli. They look remarkably like the collar cells found in sponges, and that resemblance is no coincidence: animals and choanoflagellates share a common ancestor.
Comparative genomics has been especially revealing. By comparing the genomes of animals and choanoflagellates, researchers have found that their last common ancestor already possessed molecular tools once thought to be animal-exclusive. For instance, that ancestor encoded at least three copies of a gene family involved in building cilia, along with a regulatory gene called FoxJ1.6PubMed Central. An RFX transcription factor regulates ciliogenesis in the closest living relatives of animals Other work has shown that G-protein-coupled receptors, a huge family of cell-surface signaling molecules central to everything from vision to smell in animals, are present in choanoflagellates and other close relatives of metazoans, though their functions in those organisms are still poorly understood.7PubMed Central. G-protein-coupled receptor diversity and evolution in the closest living relatives of metazoa
The picture that emerges is that the transition from single-celled life to animal multicellularity did not require inventing an entirely new molecular toolkit from scratch. Many of the genes were already in place. The evolutionary leap was in repurposing and combining those existing tools into something new: a coordinated multicellular body. The origin of metazoans from their unicellular ancestors is considered one of the most important evolutionary transitions in the history of life, and despite roughly two centuries of investigation, much about how it happened remains unresolved.8BMC Biology. Insights into the origin of metazoan multicellularity from predatory unicellular relatives of animals
The Deepest Branches of the Animal Tree
Which living animals sit closest to the base of the metazoan family tree? For most of the history of zoology, the answer seemed obvious: sponges. Sponges have no true tissues, no nervous system, no gut. They look like the simplest possible animal, and it made intuitive sense that they branched off first. But whole-genome sequencing upended that picture. Several phylogenetic analyses of genome-scale data have suggested that comb jellies (ctenophores), not sponges, are the sister group to all other animals.9PubMed Central. Extracting phylogenetic signal and accounting for bias in whole-genome data sets supports the Ctenophora as sister to remaining Metazoa
This remains one of the most contentious debates in animal evolution. The result is sensitive to which species are included in the analysis, how many genes are sampled, and which statistical methods are used.10Trends in Ecology & Evolution. The hidden biology of sponges and ctenophores If ctenophores really did branch off first, it has profound implications. Comb jellies have a nervous system, muscles, and a through-gut, while sponges have none of these. One possibility is that these complex features evolved independently in ctenophores and in the lineage leading to cnidarians and bilaterians. Another is that sponges lost traits their ancestors had, becoming simpler over time rather than staying simple from the start. Either way, the traditional idea that animal evolution was a neat, stepwise accumulation of complexity looks too tidy.
There is also the ctenophore nervous system itself, which turns out to be stranger than anyone expected. Volume electron microscopy of a comb jelly’s nerve net revealed that neurons form a continuous, fused membrane called a syncytium, fundamentally different from the separate, synapse-connected neurons seen in jellyfish and all bilaterally symmetric animals.11PubMed Central. Syncytial nerve net in a ctenophore adds insights on the evolution of nervous systems This raises the possibility that nervous systems may have evolved more than once, independently, in different animal lineages.
Body Plans and Development
One of the defining features of metazoans is that they develop from an embryo. After fertilization, cells divide and eventually undergo gastrulation, a stage in which the embryo reorganizes itself from a relatively simple ball or sheet of cells into a layered structure with distinct body axes. Gastrulation establishes the germ layers: the inner layer (endoderm) gives rise to the gut lining, the outer layer (ectoderm) to skin and nervous tissue, and in many animals a middle layer (mesoderm) produces muscle, bone, and circulatory tissue.
A traditional classification divides animals into diploblasts (two germ layers, like jellyfish and corals) and triploblasts (three germ layers, like insects and vertebrates). But the boundary between them is blurrier than textbooks suggest. Studies of the coral species Acropora millepora have found that its early development involves a process strikingly similar to gastrulation in triploblastic animals, and the genes controlling it show remarkable parallels with those governing mesoderm formation in fruit flies, hinting at deep evolutionary connections between endoderm formation in cnidarians and mesoderm formation in bilaterians.12PubMed. snail expression during embryonic development of the coral Acropora: blurring the diploblast/triploblast divide?
Among the bilaterally symmetric animals (the bilaterians, which include the vast majority of animal species), a key distinction concerns how the mouth forms during development. Protostomes (“mouth first”) develop the mouth from or near the site of gastrulation; deuterostomes (“mouth second”) develop the anus there instead, with the mouth forming as a secondary opening.13PubMed Central. Evolution of Development: The Details Are in the Entrails Protostomes include insects, mollusks, and worms. Deuterostomes include vertebrates, sea urchins, and sea stars. This seemingly minor difference in embryonic geometry reflects a profound split in the animal tree that occurred hundreds of millions of years ago.
Bilaterians also share a set of deeply conserved genes called Hox genes, which act as master regulators of body patterning along the head-to-tail axis. Changes in when and where Hox genes are turned on have been closely associated with the evolution of different body plans across the animal kingdom.14PubMed Central. Hox genes and evolution All metazoans also produce primordial germ cells, the precursors of sperm and eggs, which originate from embryonic tissues and expand through cell division.15PubMed Central. Evolving Lessons on Metazoan Primordial Germ Cells in Diversity and Development Sexual reproduction is widespread across the animal tree, though many lineages have also evolved various forms of asexual reproduction.
How Metazoans Feed
Animals are heterotrophs, meaning they cannot photosynthesize or build organic molecules from scratch the way plants do. They need to eat. But “eating” takes many forms across the metazoan tree. The most ancient feeding strategy appears to be phagocytosis, in which individual cells engulf food particles directly. This is the dominant or sole feeding mechanism in sponges, comb jellies, and cnidarians, and it persists in certain bilaterian lineages as well, making it almost certainly the ancestral feeding mode of all metazoans.16PubMed Central. A non-bilaterian perspective on the development and evolution of animal digestive systems
Over evolutionary time, many metazoan lineages developed dedicated digestive cavities and eventually through-guts, allowing food to be processed in a more controlled chemical environment and moved in one direction from mouth to anus. Some groups, like cnidarians, have a single opening that serves as both mouth and anus. Others, particularly bilaterians, evolved complete digestive tracts with specialized regions for breakdown and absorption. The diversity of feeding strategies among metazoans, from filter-feeding sponges to predatory cats, reflects hundreds of millions of years of ecological innovation built on that shared heterotrophic foundation.
The Fossil Record of Early Animals
Molecular clock studies and paleontological evidence agree that multicellular animals arose more than 600 million years ago.8BMC Biology. Insights into the origin of metazoan multicellularity from predatory unicellular relatives of animals The earliest large-scale fossil record of complex animal-like life comes from the Ediacaran period (roughly 635 to 541 million years ago). The Ediacara Biota, found on every continent, represent a diverse collection of soft-bodied organisms living in marine environments.17PubMed Central. The advent of animals: The view from the Ediacaran Many of these organisms, like the rangeomorphs, had large surface areas, lived attached to the seafloor, and could not move.18Geological Society, London, Special Publications. Ediacaran pre-placozoan diploblasts in the Avalonian biota: the role of chemosynthesis in the evolution of early animal life Some of them are genuinely difficult to classify as animals, plants, fungi, or anything else.
One iconic Ediacaran fossil, Dickinsonia, is now widely accepted as one of the oldest macroscopic metazoans in the fossil record. Abundant specimens preserved in various taphonomic states have allowed researchers to compare the biomechanical responses of Dickinsonia tissue to those of modern organisms, confirming it was a soft-bodied animal rather than a lichen or a failed evolutionary experiment outside the animal kingdom.19Geology. Stretched, mangled, and torn: Responses of the Ediacaran fossil Dickinsonia to variable forces
Then came the Cambrian explosion, roughly 541 to 485 million years ago, when the fossil record suddenly fills with recognizable representatives of modern animal phyla. The abruptness of this event is real, not an artifact of incomplete preservation. The rapid diversification of animal body plans coincided with the evolution of hard, mineralized shells and skeletons, which preserve far more readily than soft tissue.20Current Biology. The Cambrian explosion High-precision dating of Cambrian rock formations has confirmed an explosive tempo to this early radiation of modern animal phyla.21PubMed Central. Cambrian explosion condensed: High-precision geochronology of the lower Wood Canyon Formation, Nevada
Oxygen and the Rise of Animals
A long-standing idea holds that rising oxygen levels in the atmosphere and oceans triggered the origin and diversification of animals. The logic is straightforward: large, active, multicellular organisms need more oxygen than single-celled ones. The metazoan oxygen-sensing system, built around a molecular pathway called HIF (hypoxia-inducible factor), can be traced back roughly 800 million years, likely to the last common ancestor of all metazoans, and it arose when atmospheric oxygen was at roughly one percent of present levels.22PubMed Central. The origin and distribution of the main oxygen sensing mechanism across metazoans
But the relationship between oxygen and animals is not as simple as “more oxygen, therefore animals.” Laboratory work has shown that modern demosponges, which serve as analogs for the earliest animals, can survive at oxygen levels as low as 0.5 to 4 percent of today’s atmosphere. Because the last common ancestor of metazoans likely had a sponge-like physiology, its oxygen needs could have been met well before the major oxygenation events of the Ediacaran period. The origin of animals may not have been directly triggered by a contemporaneous rise in oxygen.23PubMed Central. Oxygen requirements of the earliest animals
What paleoredox data from marine sediments show is a more complex picture: ocean oxygenation during the Ediacaran and early Cambrian was highly variable in both space and time, with pockets of oxygen-rich water surrounded by a dominantly oxygen-poor ocean. This pattern of patchy, dynamic oxygenation shows a general spatial and temporal coupling to early animal diversification, but the relationship looks more like a co-evolution than a simple cause and effect.24PubMed. Heterogeneous and dynamic marine shelf oxygenation and coupled early animal evolution
MicroRNAs and Other Shared Molecular Features
Beyond the signaling pathways and structural proteins already discussed, metazoans share other molecular innovations. MicroRNAs are tiny RNA molecules that regulate gene expression by silencing specific genes after they have been transcribed. They are crucial for normal development and physiology in animals, and for a long time they were thought to have evolved independently in animals and plants. But recent evidence from early-branching metazoans and from various algae has raised the possibility that the last common ancestor of plants and animals may already have used a microRNA pathway.25PubMed Central. The evolutionary origin of plant and animal microRNAs Whether or not that is the case, microRNAs are deeply conserved across the metazoan tree and play essential roles in the fine-tuning of gene activity that makes complex animal bodies possible.
Animals That Break the Usual Rules
The definition of “metazoan” is broad enough to include some organisms that seem to violate almost every intuition about what an animal should be. The placozoans are a case in point. Trichoplax adhaerens, the best-known species, is a tiny, flat, amoeba-like creature only a few millimeters across. It has no organs, no nervous system, no gut, no body axis, and just a handful of cell types. It is among the morphologically and genetically simplest animals known, yet it is a fully fledged metazoan with an embryo, a genome, and the capacity for whole-body regeneration.26PubMed Central. Studying Placozoa WBR in the Simplest Metazoan Animal, Trichoplax adhaerens
Even more extreme are the myxozoans, a group of microscopic parasites that infect fish and other aquatic animals. For decades, myxozoans were classified as protists because they looked nothing like animals: many consist of just a few cells, with no recognizable tissues or organs. Molecular evidence eventually revealed that they are actually cnidarians, relatives of jellyfish and corals, that have undergone dramatic evolutionary simplification in response to a parasitic lifestyle. Their genomes are among the smallest of any metazoan, with major gene losses including, in one lineage, the complete loss of the mitochondrial genome.27PubMed. Myxozoans: Ancient metazoan parasites find a home in phylum Cnidaria Despite all this reduction, myxozoans have retained their cnidocysts, the stinging capsules characteristic of cnidarians, though they use them not for capturing prey but for initiating infection of a new host.
These examples are a useful corrective against defining “animal” too narrowly. Metazoans are not defined by having a brain, or a gut, or muscles, or any particular level of visible complexity. They are defined by shared ancestry and the deep molecular and developmental features that connect a placozoan to a primate across more than half a billion years of evolutionary divergence.